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Command-Line Expansion and Quoting

If you have ever typed many commands at a Unix command prompt, you will be aware that not every character you type is interpreted literally. Consider this command, for example:

[email protected]:~# echo *
Hello.txt Python-3.4.1 Python-3.4.1.tgz Python_files build gmapenv hola.txt otro text.txt virtualenv-1.11.6 virtualenv-1.11.6.tar.gz
[email protected]:~#

The asterisk * in this command was not interpreted to mean “print out an asterisk character to the screen”; instead, the shell thought I was trying to write a pattern that would match all of the file names in the current directory. To actually print out an asterisk, I have to use another special character—an “escape” character, because it lets me “escape” from the shell's normal meaning—to tell it that I just mean the asterisk literally:

[email protected]:~# echo Here is a lone asterisk: \*
Here is a lone asterisk: *
[email protected]:~# echo And here are '*' two "*" more asterisks
And here are * two * more asterisks
[email protected]:~#

The rules by which modern shells interpret the special characters in your command line have become quite complex. Instead, to use the command line effectively, you just have to understand two points:

  • Special characters are interpreted as special by the shell you are using, like bash.
  • When passing commands to a shell either locally or across the network, you need to escape the special characters you use so that they are not expanded into unintended values on the remote system.